Basic auxochromes. Auxochrome vs chromophore. Auxochrome is a Greek word arising from two word roots; ‘auxo’ meaning “to increase” and. This lesson explains the parts of a molecule responsible for color: chromophores and auxochromes. Chromophores and auxochromes are. In , Witt proposed that dye molecules contain two groups; the chromophore and the auxochrome. The chromophore is a group of atoms which control the.

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The electromagnetic spectrum has a very wide variation in wavelengths but the human eye visualizes only short wavelength radiation. The electrons which were in the resting state are elevated into the excited state because of the energy incorporated into it. Dyes containing auxochromes are basically aromatic compounds and include the presence of aryl rings which have delocalized electron systems.

Auxochrome is a group of atoms which is functional and has the capability to alter the capacity of the chromophore to reflect colors. Halochromism occurs when a substance changes color as the pH changes. Normally, auxochromes which intensify the colour are chosen. Principles of Organic Chemistry. Colour due to refraction, scattering, dispersion and diffraction- these are all due to the geometrical and physical dimensions of a solid or a solution.

At that time, Witt suggested that the auxochrome was a salt-forming group, which helped to improve the colour of the dye.

Chromophore – Wikipedia

The chromophore is the part of the molecule where the energy difference between two different molecular orbitals falls within the range of the visible spectrum and hence absorbs some particular colours from visible light. His theory was later modified when it was discovered that the chromophore is usually electron-withdrawing, and auxochromes are normally electron-donating.

The hue of a dye depends on the wavelength it absorbs. Auxochromes are molecules which are attached to non ionizing compounds yet retain their ability to ionize and affect the ability of absorbing light when attached to a chromophore. So how can a dye molecule’s hue be altered? The reverse shift, towards shorter wavelengths is known as a hypsochromic effect 2. For instance, benzene does not have any color of its own, but when it is combined with the -nitro group which acts as a chromophore; it imparts a pale yellow color.


Simple molecular excitation, such as in a neon tube, may cause the appearance of colour. The first four mechanisms all rely on some form of energy transfer to move either molecules or electrons from their ground state into some excited state. The overlapping p -orbitals effectively mean that no one electron absorbs more energy than another, since all p -electrons in the conjugated system are smeared above and below the molecule.

Chromophores with double covalent bond appear to be colored as a result of the heightened state of the electrons. However, only one of these effectively applies to dye molecules, since dye molecules are almost without exception organic conjugated systems.

The different colours of white light Everyone is familiar with rainbows- see the top picture for a well known example!

Since the wavelengths the dye absorbs depends on its structure, we can see that any change which affects the p -system will affect the hue. Electronic motion in conjugated organic systems, and charge transfer.

Similar behavior is seen in azobenzene which has a red color, but p -hydroxyazobenzene is dark red in color. By using this site, you agree to the Terms of Use and Privacy Policy. There is no need to resubmit your comment.


Chemical compounds Color Chemical reactions. Examples of such compounds include bilirubin and urobilinwhich exhibit a yellow color. An auxochrome is known as a compound that produces a bathochromic shiftalso known as red shift because it increases the wavelength of absorption, therefore moving closer to infrared light. Views Read Edit View history. The superposition of these determines how broad an absorption band is 2. Chromophore is that part of the molecule which when exposed to visible light will absorb and reflect a certain color.


User assumes all risk of use, damage, or injury. Leave a Response Cancel Reply Name required. This change in structure affects a chromophore in the pH indicator molecule.

This is due to other wavelengths absorbed by the dye, other than the one which causes the hue. Retrieved from ” https: Color space Color model additive subtractive Color mixing Primary color Secondary color Tertiary color intermediate Quaternary color Quinary color Aggressive color warm Receding color cool Pastel colors Color gradient.

They themselves fail to produce the colour; but when present along with the chromophores in an organic compound intensifies the colour of the chromogen. Notify me of amd comments via e-mail. Retrieved from ” https: Various factors in a chromophore’s structure go into determining at what wavelength region in a spectrum the chromophore will absorb. A structural change which causes the absorption band to longer wavelengths i.

chemistry of dyes

Colour in crystalline solids arises from band theory- the blurring of many orbitals through-out the solid. G—M List of colors: Linguistic relativity and the color naming debate Blue—green distinction in language Color history Color in Chinese culture Traditional colors of Japan Human skin color.

Light waves with frequency very close to their natural frequency are absorbed readily. Views Read Edit View history. In order to prepare dyes, auxochromes are attached to chromophores so as to achieve the deep color intended for the product. Category Portal Index of color-related articles.

The two groups are connected by a conjugated system 2. In biological molecules that serve to capture or detect light energy, the chromophore is the moiety that causes a conformational change of the molecule when hit by light.